The permissible temperature limit for the winding of a motor is primarily determined by the motor's insulation. Applicable standards (UL, CSA, IEC, and NEMA) distinguish different classes of insulation and corresponding temperature limits. Typical allowable temperatures inside the stator windings are given in Information Guide for General Purpose Industrial AC Small and Medium Squirrel-Cage Induction Motor Standards, NEMA Standard MG1-2003, August 2003.
If the motor is working continuously in an over-temperature condition, the aging process is accelerated. This is a chemical process that involves the deterioration of the insulation material. It is often assumed that a winding temperature that is constantly 10° C. higher than the temperature limit reduces the motor life by half. This life law shows that particular attention must be paid to adhering to the permitted operating temperature for long periods of time.
Various standards have been established to provide general guidelines in estimating stator winding temperature for motor overload protection. See, e.g., IEEE Guide for AC Motor Protection, IEEE Standard C37.96-2000, March, 2000; Guide for the Presentation of Thermal Limit Curves for Squirrel Cage Induction Machines, IEEE Standard 620-1996, June, 1996; and IEEE Standard Inverse-time Characteristic Equations for Overcurrent Relays, IEEE Standard C37.112-1996, September, 1996.
Many manufacturers provide motor protective relays based on a thermal model with a single thermal time constant, derived from the temperature rise in a uniform object.